The present invention relates generally to the field of implantable medical devices, and more particularly to low energy uplink and downlink telemetry control for an implantable medical device (IMD) telemetry transceiver.
At present, a wide variety of IMDs are commercially released or proposed for clinical implantation that are programmable in a variety of operating modes and are interrogatable using RF telemetry transmissions. Such medical devices include implantable cardiac pacemakers, cardioverter/defibrillators, cardiomyostimulators, pacemaker/cardioverter/defibrillators, drug delivery systems, cardiac and other physiologic monitors, electrical stimulators including nerve and muscle stimulators, deep brain stimulators, and cochlear implants, and heart assist devices or pumps, etc.
Typically, certain therapy delivery and monitoring operational modes and parameters of the IMD are altered temporarily or chronically in a non-invasive (i.e. non-surgical) manner using downlink telemetry transmission from an external programmer of programming and interrogation commands (herein referred to as xe2x80x9cdownlink telemetry dataxe2x80x9d). Moreover, a wide variety of real time and stored physiologic and non-physiologic data (referred to collectively herein as xe2x80x9cpatient dataxe2x80x9d) is uplink telemetered by the IMD to the programmer in response to a downlink telemetered interrogation command.
The telemetry transmission system that has evolved into common use currently relies upon the generation of low amplitude magnetic fields by current oscillating in an LC circuit of an RF telemetry antenna in a transmitting mode and the sensing of currents induced a closely spaced RF telemetry antenna in a receiving mode. Short duration bursts of the carrier frequency using a variety of telemetry transmission and encoding formats are transmitted through the patient""s skin between the antennae and transceiver circuits in a programming head overlying the skin and the IMD under the skin. In the current MEDTRONIC(copyright) product line, the RF carrier frequency is set at 175 kHz and the RF telemetry antenna of the IMD is typically coiled wire wound about a ferrite core that is located within the hermetically sealed enclosure. The hermetically sealed enclosure also typically contains a battery power source and circuitry for controlling the operation of the IMD and a reed switch or MAGFET that is responsive to an externally applied magnetic field within the external programming head to enable decoding of downlink telemetry transmissions by and transmission of uplink telemetry from the IMD.
In an uplink telemetry transmission from an IMD, it is desirable to limit the current drain from the IMD battery as much as possible simply to prolong IMD longevity. As the technology advances, IMDs become ever more complex in possible programmable operating modes, menus of available operating parameters, and capabilities of monitoring increasing varieties of physiologic conditions and electrical signals. These complexities place ever increasing demands on the programming and interrogation system and the medical care providers using them. Thus, as device operating and monitoring capabilities multiply, it is desirable to be able to transmit out ever increasing volumes of data in real time or in as short a transmission time as possible with high reliability and immunity to spurious noise. Moreover, it is desirable to eliminate the need for the magnetic field coupling between the programming head and the IMD and to allow secure programming and interrogation to take place at greater distances between the IMD and programmer antennae.
As a result of these considerations, many RF telemetry transmission data encoding schemes have been proposed or currently are used that increase security and the data transmission rate as well as the safe operating distance between the IMD and programmer antennae. One way to increase data transmission capacity is to increase the RF carrier frequency and the bandwidth allocated to an active transmission channel into the MHz range as set forth in commonly assigned U.S. Pat. No. 5,861,019 and in pending U.S. patent application Ser. No. 09/302,932 for a xe2x80x9cTelemetry System for Implantable Medical Devicesxe2x80x9d, filed Apr. 30, 1999, by Villesca et al.
The above-referenced 175 kHz RF carrier frequency is generated employing a relatively simple low current consuming L-C tank circuit and switching circuitry. But, a high frequency RF generator is necessary to generate the high frequency RF carrier signal in the MHz range, and it is necessary to carefully control the generator to prevent frequency drift without unduly increasing current consumption from the IMD battery.
Similar problems exist in other non-IMD communication systems operating with a particular RF carrier frequency or within particular allocated frequency bands in FM transmission and reception modes as set forth in U.S. Pat. Nos. 4,521,918, 4,955,075, 5,335,365, 5,748,103, 5,767,791 and 5,944,659, for example. Typically, a battery powered remote device, e.g., an external patient monitor or a mobile cellular phone, is powered by a battery and communicates with remote, line powered equipment either periodically, in the case of a monitor, or, in the case of a cellular phone, when a user answers an incoming call or initiates an outgoing call. The battery powered monitor or cellular phone employs a frequency synthesizer to generate the RF carrier signal during transmission of data or voice, and the frequency synthesizer typically comprises a voltage controlled oscillator (VCO) and a phase lock loop (PLL) circuit that regulates the frequency of the generated RF signal. The PLL circuit operates in a feedback path employing a reference frequency to develop a PLL control voltage maintained on a capacitive loop filter that is applied to a control input of the VCO which responds by oscillating at the RF carrier frequency established by the control voltage. In the transmission mode, the RF carrier frequency is modulated in frequency by the superimposition of a data or voice voltage on the control voltage, thereby increasing or decreasing the VCO generated carrier frequency.
The PLL circuit consumes battery energy, and so, it is often only operated to stabilize the VCO and is then turned off during data or voice transmission or during a standby mode, as suggested in the above-referenced ""365 patent. In addition, it is proposed in the above-referenced ""075 patent to employ automatic frequency control (AFC) during reception of the RF carrier frequency of a received signal to stabilize the VCO frequency. In the receive mode, the VCO frequency is initially stabilized by the PLL circuit, and then the AFC is substituted for the PLL, which is disconnected from the VCO and/or powered off.
It is also proposed to remove power from the PLL circuit or disconnect it from the VCO during the transmission mode after the VCO voltage has stabilized to within acceptable frequency tolerances and provides the control voltage on the capacitive loop filter. However, the control voltage stored by the loop filter tends to is decline due to current leakage over time, and so it is necessary to periodically power up and/or reconnect the PLL circuit to the loop filter and VCO to restore the control voltage as described in the above-referenced ""918 patent. Or, the control voltage that is developed in a transmit or receive mode is stored and is used during the standby mode to maintain the control voltage via a feedback loop under the control of a microcomputer as described in the above-referenced ""365 patent. The feedback loop employs A/D and D/A converters and is not used during the transmit or receive modes because it would inherently introduce noise on the transmitted or received signal. The circuitry of such a feedback loop also consumes space on the RF module that must be fitted into the limited space within the IMD housing.
Accordingly, it is an objective of the present invention to save IMD battery energy during telemetry sessions while still generating the high RF carrier frequency that is required to provide the data transmission rate required between the IMD and the external programmer at a distance from the patient and also meets the standards established by regulatory agencies for accuracy, stability and patient safety.
In accordance with the present invention, the frequency synthesizer employed in the RF transceiver of the IMD operating system functions in a PLL LOCK mode wherein the VCO frequency is governed by the PLL and an energy saving HOLD mode wherein the PLL is not operational, and control voltage dissipation is compensated for during uplink and downlink telemetry transmissions.
The RF transceiver is normally dormant and powered down until an event occurs that signifies the start of a telemetry session, the telemetry session involving operating the IMD and the external programmer in successive uplink and downlink telemetry transmissions. The PLL circuit is powered up and coupled with a control voltage input and the output of the VCO to develop a frequency control voltage stored by a capacitive loop filter during initial LOCK portions of both uplink (transmitting) and downlink (receiving) telemetry transmission time periods. The VCO also has a frequency modulation (FM) input that receives the data bit modulation voltage that modulates the carrier frequency during uplink transmission of patient data. The PLL circuit is disconnected from the VCO and the loop filter and is placed in a low energy state during the subsequent HOLD portions of both uplink and downlink telemetry transmission time periods when the control voltage and the resulting VCO carrier frequency have stabilized sufficiently.
During the HOLD portion of a downlink telemetry transmission, the VCO generated carrier signal and the received signal are mixed, and the telemetered information of the RF signal that is modulated and transmitted by the programmer is demodulated. An AFC algorithm is enabled during the HOLD portion of the downlink telemetry transmission and derives a frequency correction value from the difference in frequency of the average frequency of the received carrier frequency and the VCO generated carrier frequency. The frequency correction value is applied to the VCO to compensate for VCO frequency tolerances and for loop filter capacitor discharge of the control voltage to thereby drive the VCO generated carrier frequency toward the average carrier frequency of the received carrier signal.
The frequency correction value derived by the AFC algorithm is converted to a frequency correction voltage value that is applied to the FM input of the VCO. The correction voltage value is effectively summed with the decreasing loop filter capacitor voltage by the VCO, and the VCO responds to the summed voltages to generate the VCO carrier frequency so that the VCO generated carrier signal remains relatively constant over the HOLD portion while receiving a downlink telemetry transmission. Each successively determined correction voltage value increases with time in as the control voltage stored in the loop filter capacitor discharges.
Preferably, drift of the VCO generated carrier frequency during the HOLD portion of the uplink telemetry transmission time period is compensated for through use of the frequency correction values developed in a preceding HOLD portion of a downlink telemetry transmission. The frequency correction values periodically developed by the AFC algorithm during the HOLD portion of a downlink telemetry transmission are processed by a transmit drift compensation circuit. The stored frequency correction values of the data set are each successively retrieved and converted to a correction voltage value that is summed with the modulation voltage of the data signal that is applied to the FM input of the VCO. In this case, the sum of the correction voltage value and the modulation voltage of the patient data signal (when present) is effectively summed with the decreasing loop filter capacitor voltage by the VCO. Again, the VCO responds to the summed voltages applied to the two inputs to generate the VCO carrier frequency so that the VCO generated carrier signal remains relatively constant over the HOLD portion of the uplink telemetry transmission.
In a further preferred embodiment, a fixed recharge current is applied by a recharge current source to the loop filter capacitor to compensate for voltage discharge during both the LOCK and HOLD portions of each uplink and downlink telemetry transmission time period. The fixed recharge current value that is derived and stored in IMD memory tends to recharge the loop filter capacitor toward the required control voltage and thereby compensates for current leakage. In one variation of this embodiment, the rate of change of the correction voltage values that are derived by the AFC algorithm during the HOLD portion of the downlink telemetry transmission time period is calculated and stored in IMD memory as a fixed recharge current value. In another variation of this embodiment, the rate of capacitor discharge of the loop filter capacitor over time is measured following assembly of the IMD, and the recharge current value is derived and stored in memory as a function of the rate of capacitor discharge.
Preferably, both the relatively coarse recharge function of the recharge current source and the fine correction functions enabled by the AFC algorithm and applied in real time or retrieved from memory in downlink and uplink telemetry transmission time periods, respectively, are employed together.
This summary of the invention and the objects, advantages and features thereof have been presented here simply to point out some of the ways that the invention overcomes difficulties presented in the prior art and to distinguish the invention from the prior art and is not intended to operate in any manner as a limitation on the interpretation of claims that are presented initially in the patent application and that are ultimately granted.